Integrated circuits can be fabricated on a semiconductor wafer using a photolithography process. As features sizes of the integrated circuits become smaller, it is important to control the uniformity of the critical dimensions of resist features on the wafer. The critical dimensions refer to the dimensions of critical features on the wafer, such as minimum line width of single lines, minimum line width of densely spaced lines, and the minimum size of contact holes. The critical dimension uniformity greatly affects the yield and performance of integrated circuits produced from each wafer. The critical dimensions on the wafer can be measured using, for example, scanning electron microscopy. When non-uniformity in the critical dimensions within the image field on the wafer is detected, the photolithography system can be adjusted to reduce the non-uniformity to improve the quality of the integrated circuits fabricated on the wafer. The quality of integrated circuits can also be improved by reducing registration and overlay errors. Registration errors occur when features on a mask are offset from their intended positions. Overlay errors occur when features from two or more masks that are designed to be aligned with each other are not properly aligned during the lithography process.